I know verb-んじゃない can be used in colloquial speech as a negative imperative (e.g. するんじゃない) in lieu of verb-な (するな), but why exactly does it function that way? That is, what is the rationale/etymology behind v-んじゃない being a negative imperative?

  • FWIT, the の is not compulsory.
    – oldergod
    Jul 27 '16 at 7:22
  • @oldergod, I would say that it is for this meaning, at least in Standard Japanese.
    – dainichi
    Jul 28 '16 at 3:32
  • @dainichi I have seen it missing many times for the negative command, as broccoli, kindly confirms.
    – oldergod
    Jul 29 '16 at 0:11
  • @oldergod, ~でない without の is fine, ~じゃない without の is not.
    – dainichi
    Jul 29 '16 at 0:18

Obviously it's not grammatical imperative, but the construction functions as order when used by somebody's betters (senior, superior etc.) to strongly admonish them. If I can ignore context, "You don't want to do —!" could be a way of translation.

  • V(する)-んだ! (more pompously V(する)-のだ!): affirmative command
  • V(する)-んじゃない! (V(する)-のではない!, V(する)-でない!): negative command

Incidentally, the dictionary form is also employed as command, with colloquial but very overbearing tone.

  • V(する)!: affirmative
  • V(し)-ない!: negative
  • Rather than "You don't want to do...", I think maybe "You aren't ..." is a more literal translation of this.
    – Locksleyu
    Jul 27 '16 at 14:44

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